Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is referred to as an ion implantation recipe.
FIG. 1 depicts a prior art ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target substrate 118. For illustration purposes, these components are often referred to as “beam-line elements.”
In production, semiconductor wafers are typically scanned with an ion beam. For example, as illustrated in FIG. 2, a ribbon-shaped ion beam 202 may be kept stationary while a series of wafers 204 may flow along a line 20 and across the ribbon-shaped ion beam. Alternatively, as illustrated in FIG. 3, a spot beam 302 may be scanned back and forth between two endpoints 308 and 310 forming a beam path 30, while a series of wafers 304 may flow along a line 32 across the beam path 30. As used hereinafter, “scanning” of an ion beam refers to the relative movement of an ion beam with respect to a wafer or substrate surface.
In a traditional ion implanter system, an ion beam is typically tuned to have a specified incident angle on a substrate surface, and any spread of the incident angle of the single ion beam is usually minimized or simply ignored. However, in reality, the ion beam does not always strike a target substrate exactly at the specified angle, and the ion beam often has a finite angle spread that is not negligible. As shown in FIG. 4a, a ribbon-shaped ion beam 400 typically comprises a plurality of beamlets 404. Due to beam emittance and/or divergence, the beamlets 404 may strike a substrate surface 402 at different incident angles. Thus, the substrate surface 402 is exposed to an intrinsic spread of ion beam incident angles. Further, as shown in FIG. 4b, each beamlet 404 may also have an intrinsic spread of incident angles due to space-charge effect, for example. That is, ions that form the beamlet travel in an average direction but spread out according to a Gaussian-like distribution around the average direction. Similarly, a typical spot beam may also have an intrinsic angle spread, and, due to beam steering errors, the spot beam may not strike its target at exactly the specified incident angle.
Ion beam incident angles and intrinsic angle spread may cause angle variations in the ion implantation process. There are typically three types of angle variations, whose causes and effects are illustrated in FIGS. 5-7 respectively.
FIGS. 5a and 5b illustrate wafer-to-wafer (or inter-wafer) angle variations, wherein wafers 502 and 504 are different wafers separately processed based on the same recipe in the same ion implanter system. Due to small differences in the setups of the ion implanter system and/or beam steering errors, the wafer 502 may be implanted with an ion beam 50 incident at a first angle θ, while wafer 504 may be implanted with an ion beam 52 incident at a second angle θ′, wherein θ′≠θ. θ and θ′ are “angle errors” measured with respect to a nominal direction of the wafer surface. In the description that follows, the angle errors are shown as being measured with respect to the normal incidence of the wafer surface. However, in general, such angle errors may be measured with respect to any predetermined angle. The angle errors typically affect all structures on the wafers 502 and 504, and the angle difference can cause wafer-to-wafer variations in device performance. The ion beams 50 and 52 may also have different intrinsic angle spreads which may cause additional doping variations between the two wafers.
FIG. 6 illustrates within-wafer (or intra-wafer) angle variations, wherein different parts of a wafer 602 may experience different ion beam incident angles (θ1, θ2, and θ3, etc.) due to intrinsic angle spread within an ion beam 60, for example. Alternatively, a wafer with an irregular surface (e.g., concave or convex surface) may have significant intra-wafer angle variations even when it is exposed to a perfectly parallel ion beam (i.e., an ion beam with zero angle spread). While the beam current non-uniformities may be averaged out by scanning the ion beam across the wafer, for example, the ion beam incident angles in different parts of the substrate may remain uncontrolled such that the angle spread is narrow locally (i.e., at any part of the substrate) but still varies from one location to another. Such intra-wafer angle variations can cause significant performance variations for devices located in different parts of the same wafer.
FIG. 7 illustrates device-level angle variations. As shown, a first ion beam 70 and a second ion beam 72, with or without an angle error, may cause a trench 702 or a mesa 704 to see a spread of incident angles. As a result, the bottom of the trench 702 may have a different dopant profile from its sidewalls. And each sidewall of the trench 702 may have a different dopant profile from another. Similarly, the mesa 704 may have one side more heavily doped than an opposite side. For certain applications, such an asymmetrical dopant profile may not be acceptable.
The above-described angle variations may cause a number of problems if the ion beam incident angles and/or angle spread are not properly controlled in the implantation and doping processes.
One such problem may arise in the context of “conformal doping,” where a uniform dopant profile is desired in a substrate with irregular surface topology. Prior methods for conformal doping start by depositing a dopant-containing film on a substrate surface. Then, some post-implant processing, such as thermal diffusion, is required to drive the dopants into the substrate. To achieve a uniform dopant profile, the prior methods typically focus on improving the uniformity of the thermal drive-in process. Since such methods rely on thermal diffusion, they are limited by thermal budget constraints for each doping step in the process sequence.
FIGS. 8a and 8b illustrate another problem that may be caused by ion beam angle variations. FIG. 8a depicts an ion beam 80 with zero angle error and a small angle spread. The ion beam 80 is used to dope a substrate surface 802, a part of which is masked by a structure 804 (e.g., a gate stack) having vertical sidewalls. Since the ion beam 80 is aligned with the sidewalls, the resulting dopant profiles 82 and 84 on either side of the structure 804 are symmetric. If, however, the ion beam 80 has a small angle error as shown in FIG. 8b, the shadowing effect from the structure 804 causes the resulting dopant profiles 86 and 88 to be highly asymmetrical to the extent that the shadowed side becomes useless.
Note that the structure 804 may be just one of the devices in the substrate 802 whose topology makes it sensitive to ion beam angle variations (e.g., beam steering errors and angle spreads). If the angle error and/or angle spread of the ion beam 80 are not properly controlled, similar yet varying effects may be seen across different parts of the substrate 802 or across different wafers. As the device feature size continues to shrink, the device-level, wafer-level, and wafer-to-wafer angle variations, if left uncontrolled, may cause more performance variations and other detrimental effects.
The ion beam angle variations may also cause process repeatability problems in an ion implanter system. As described above, uncontrolled ion beam incident angles and angle spread may cause significant performance variations among different wafers processed in the same implanter. Existing methods for setting up an ion implanter system have been focused on the repeatability of implant doses. In terms of ion beam incident angle, the existing approach has been limited to the correction of average incident angles only. No known method manages to achieve a true process repeatability with respect to ion beam incident angles as well as implant doses.
In view of the foregoing, it would be desirable to provide a solution for ion beam implant control which overcomes the above-described inadequacies and shortcomings.